pgsql: Add a \setenv command to psql.

Add a \setenv command to psql.

This can be used to set (or unset) environment variables that will
affect programs called by psql (such as the PAGER), probably most
usefully in a .psqlrc file.

Andrew Dunstan, reviewed by Josh Kupershmidt.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/e6d9e2106f0dda459063126d07967df197b7b5fe

Modified Files
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doc/src/sgml/ref/psql-ref.sgml | 18 ++++++++++++++++
src/bin/psql/command.c | 45 ++++++++++++++++++++++++++++++++++++++++
src/bin/psql/help.c | 3 +-
3 files changed, 65 insertions(+), 1 deletions(-)